Thermal interface adhesives are essential for proper operation of electronic assemblies. They are often used between surface mount components and printed wiring boards, and between printed wiring boards and metal heat sinks. Their main function is to bond the components together, and allow good heat transfer between the parts that are bonded. Typically, they should exhibit a combination of good adhesion, good heat transfer, compliance to accommodate any mismatch in coefficient of thermal expansion (CTE) of the parts being bonded, flow control to obviate adhesive contamination onto adjacent sensitive hardware features, and rework capability. In certain cases an electrically insulating, thermally conducting interface material is needed, whereas in other cases an electrically and thermally conducting material is required. The appropriate filler material in the interface adhesive layer must be selected.
Materials normally used for this application include thermosetting resins filled with a thermally conductive filler. For example, epoxies filled with silver or aluminum oxide. One presently available material is a thermoplastic film coated on both sides with a filled thermosetting adhesive. This system is typically very rigid after curing, and does not accommodate materials having widely different coefficients of thermal expansion (CTE). This approach also does not permit removal of the cured adhesive. Moreover, the thermal conductivity of the bond layer is not very high because the thermoplastic does not include a thermally conductive filler.
In order to apply these materials, it is important to control the flow characteristics of the material during cure. Generally, the material will go through a period of very low viscosity while it is being heated to the final cure temperature, but before it is fully cured. At this time there may be excessive flow of the material into undesired areas, and/or separation of the conductive filler from the resin phase. Neither outcome is desirable.
Thus, there is a need in the art for adhesive systems that offer rework capability in combination with good adhesion, good heat transfer, and that maintain good performance over a wide temperature range.